Antialiasing in the context of digitizing line art and certain graphical image structures is best known as a method of using intermediate levels of intensity to achieve subpixel position of edges for several reasons including reduction or elimination of jaggies on the edges of lines and polygons, including text. Jaggies are, primarily visible at the edges of sloped lines approaching horizontal or vertical. The term antialiasing suggests an analog term aliasing; normally representing the presence of low frequencies resulting from sampling high frequency signals at too low a sampling rate.
Consider a near-vertical (or near-horizontal) line segment. To be perfectly reproduced in a printed media, the phase, which represents the location of the edge, must continuously vary along the length of a segment. Due to the inherent sampling of a bi-level display or printed output, the phase exhibits jump discontinuities. Thus, this form of aliasing artifact, leads to an induced jagged appearance where the structures are referred to jaggies. Within a sampled image any graphical object is eventually approximated as a polygon or collection of polygons. These polygons have straight edges some of which will exhibit aliasing (jaggies and other placement defects). FIG. 1 for example shows aliasing in two dimensions. When the triangle on the top of FIG. 1 is rasterized, the edges are aliased as reproduced in the triangle shown at the bottom of FIG. 1. In particular, the position along the bottom edge should move up slightly from column to column as one looks from left to right in the image at the bottom of FIG. 1. However, the position is quantized, as illustrated, producing the jagged appearance along the bottom of the triangle. Visibility of the anti-aliased image artifacts is increased by the regular nature of the jaggies, again a result of sampling.
Consider the following systems and their capability, or incapability, to utilize antialiased pixels. Xerox's Docucolor 40, for example, employs a high frequency analog line screen to render antialiased pixels, but that is not an option for some products or marked segments. When conventional screens (e.g., approximately equal to 130-150 CPI dots) are employed in a rendering module, antialiased pixels are halftoned and printed, resulting in objectionable halftone dots positioned along character edges. Hyperacuity printing techniques, for example those described by Curry, et al. (U.S. Pat. Nos. 5,138,339 and 5,485,289) can provide rendering for antialiased pixels that is compatible with simultaneously printing dot screen halftones in enhanced line art. However, these techniques require the use of tags to identify the antialiased pixels as antialiased line art. In the preferred embodiments described with respect to the present invention the rendering architecture distinguishes binary text/line from contone images to appropriately treat both image types. As will be described herein an algorithm or method may be employed in a rendering module or in other components of the rendering device to produce gray antialiased pixels of a form suitable for Xerographic printing.
Antialiased images can be generated by capturing the image at a resolution greater than the final or desired output resolution, then reducing the resolution of the image by sub-sampling using an averaging process. A major benefit of antialiased images is that high contrast, saturated objects are surrounded with pixels possessing intermediate values that visually suggest the true, higher resolution position of object edges.
For example, in binary printing systems, such as many xerographic or ink jet systems that use a halftoning process to simulate continuous tone images, these antialiased edge pixels should be rendered with a very high frequency cell, ideally one having the resolution of the final output image. If the standard system halftone dot were to be used, the antialiased edges would be serrated or jagged at the standard halftone frequency. This rendering would reduce or even negate any value obtained through antialiasing. The use of a very high frequency screen over the entire antialiased image renders the antialiased pixel properly, but tends to sharpen the tonal curve and provoke print quality defects in the overall image.
Hence, the present invention is directed to a logic-based method for generating antialiased rendering tags within an architecture designed for the rendering of antialiased text or line regions, and to thereby enable the antialiased pixels to be rendered in a manner distinguishable from that applied to continuous tone portions of an image.
Heretofore, a number of patents and publications have disclosed information relevant to antialiasing, the relevant portions of which may be briefly summarized as follows:
U.S. Pat. No. 5,646,751 to Motamed et al., issued Jul. 8, 1997, teaches a method for improving the speed of a color conversion operation using pixel tagging.
In "A Comparison of Antialiasing Techniques," IEEE CG&A, Vol. 1, No. 1, January 1981, pp. 40-48, F. Crow teaches that prefiltering is a computationally effective technique for antialiasing.
In accordance with the present invention, there is provided a method for the detection of antialiased pixels within rasterized image data, comprising the steps of: storing, in a scanline buffer memory, a predefined number of scanlines of data including data for a target pixel position; storing, in a tag buffer memory, a plurality of binary antialiasing tags, including at least a previously estimated tag for the target pixel position; and generating, as a function of the pixel data for the target pixel and subsequent neighboring pixels and the previously estimated tag for the target pixel position, an antialiasing tag for the target pixel position.
One aspect of the invention deals with a basic problem in the rendering of antialiased images, particularly text and line art regions--identifying the antialiased regions and then the appropriate technique for rendering the antialiased regions--in order to obtain acceptable edge quality. This aspect is further based on the discovery of a technique that alleviates this problem. The technique employs a memory efficient pixel tagging architecture that distinguishes between image types (binary text or lines and continuous tone) and determines whether a given pixel has been antialiased. Previous tagging methods have required five scanline buffers to accurately tag thin lines that are gray due to antialiasing. The present invention detects thin anti-aliased lines and anti-aliased edges using as little as two scanlines of pixel buffering in conjunction with a scanline of binary tags. The method employs memory for temporarily storing a scanline of tags. In effect, tag values are generated by examining the pixel twice, first to tag vertical lines, edges, and possible horizontal lines, and second to confirm the tagging after the advance of a scanline.